1. Field of the Invention
This invention pertains to the field of collinear and non-collinear retroreflectors and to methods for their operation.
2. Description of Related Art
Multipass optical cells have been used for many years in the field of gas and atmospheric absorption spectroscopy so that a small volume can be interrogated by making multiple passes through the volume. This achieves a long optical path length in a very compact structure, or similarly, amplifies via multiple passes a weak optical response of a single pass.
Retroreflection of an optical signal is very important in optical systems since it not only reflects the light, but also causes the return optical path to be parallel to the incident path. This has many applications such as optical delay line scanning, optical alignment, and bright passive reflections such as road signs, to name a few.
Combining the trait of a multipass optical cell with a retroreflector creates a flexible tool for use in compact laser amplifier gain stages, communication, spectroscopy, and remote sensing.
Donald R. Herriott and Harry J. Schulte in an article entitled “Folded Optical Delay Lines” in the journal Applied Optics, for August 1965, vol. 4, No. 8, starting on p. 883, disclose that a long optical path has been folded between two 7.5 cm diameter spherical or aspherical mirrors to provide an output optical beam which can be well separated from previous reflections with 1000 or more passes between the mirrors. The 3000-m path provides 10 μsec of delay. This system can be used as a dispersionless optical delay line for use in filtering or storage of information modulated onto the light beam. The pattern of beams between the two mirrors is obtained in one of two ways. A small perturbing mirror may be inserted to give a series of offset ellipses, or one or both of the mirrors can be made astigmatic to give a Lissajous pattern of spots on each mirror. The output beam can be separated from others by discriminating in both angle and position. The diffraction losses of the system are much lower than those for an open beam because of the periodic focusing of the spherical mirrors. The extreme dependence of the loss of the delay line upon the absorption and scattering loss of the mirrors makes the system a suitable method for measuring mirror loss.
U.S. Pat. No. 5,973,864 to Lehman et al discloses a stable resonator for a ring-down cavity spectroscopy cell having an optic axis. The resonator includes two Brewster's angle retroreflector prisms, each having a plurality of total internal reflection surfaces. The prisms are disposed in alignment along the optic axis of the resonator. One or both of the prisms can be rotated so that light rays enter a surface of the prism nearly at Brewster's angle to the normal of the prism surface. This feature maintains alignment between the prisms and allows the resonator to be tuned. One of the internal reflection surfaces of at least one of the prisms may be a curved surface. Alternatively, an astigmatic lens may be centered in one arm of the resonator and tilted at Brewster's angle with respect to the optic axis of the resonator.